I've always wanted to know that.
Falling under the "science that we're not real sure we can use, but darned if it isn't interesting" category, some Harvard boffins with good high-speed video and X-ray motion cameras have answered the question of how a dog's drinking mechanics works.
This footage shows that as the tongue touches the surface of the water, the liquid adheres to it, creating a water column as the tongue is drawn back towards the mouth. The dog then snaps its mouth closed just as the water begins to fall backwards towards the bowl.
Go click on the short video-- it's pretty cool.